Vehicle control by pitch modulation

ABSTRACT

A method for fore-aft stabilization of a vehicle for motion in a specified direction over an underlying surface. The vehicle has at least one forward wheel and at least one aft wheel, and the forward wheel is characterized by a force normal to the instantaneous direction of motion of the vehicle. A motor actuator drives each aft wheel, and a controller governs the motor actuator or motor actuators in such a manner as to dynamically stabilize the vehicle, according to a uniform control law, when the forward wheel is in contact with the underlying surface or not. A torque is applied to the aft wheel on the basis of vehicle pitch or the force on the forward wheel normal to the direction of motion. Additionally, a periodic rotational modulation may be applied to the aft wheel, and a stabilizing torque provided based on a detected response, either of vehicle pitch or of normal force on the front wheel. Left and right motor actuators may independently control left and right aft wheels to continue turns as governed by user steering, whether or not forward wheels are in contact with the ground.

PRIORITY

The present application claims priority from the U.S. Provisional PatentApplication 60/617,244, which is hereby incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present invention pertains to methods for actively maintainingstability and control of the motion of a vehicle equipped with one ormore forward wheels and one or more aft wheels, whereby balancedoperation may be enabled in case the front wheels lose, or are removedfrom, contact with the ground.

BACKGROUND OF THE INVENTION

Human transport devices serve to move a person over a surface and maytake many different forms. For example, a human transport device, as theterm is used herein, may include, but is not limited to, wheelchairs,motorized carts, all-terrain vehicles, bicycles, motorcycles, cars,hovercrafts, and the like. Some types of human transport may includestabilization mechanisms to help ensure that the device does not fallover and injure the user of the transport device.

A typical four-wheeled wheelchair contacts the ground with all fourwheels. If the center of gravity of the combination of the wheelchairand the user remains over the area between the wheels, the wheelchairshould not tip over. If the center of gravity is located above andoutside of the ground contacting members of the transport device, thetransport device may become unstable and tip over.

Referring now to FIG. 1A, a typical wheelchair 100 is shown. Thewheelchair 100 and the user 102 define a frame. The frame has a centerof gravity 104 located at a position vertically disposed above thesurface 106. The term “surface” as it is used herein refers to anysurface upon which a human transport device may sit or locomote.Examples of a surface include flat ground, an inclined plane such as aramp, a gravel covered street, and may include a curb which verticallyconnects two substantially parallel surfaces vertically displaced fromone another (e.g., a street curb).

The surface 106 may be at an incline as compared to the horizontal axis108 (which is a line in the plane transverse to the local vertical). Theangle by which the surface 106 is offset from the horizontal axis 108 iscalled the surface pitch and will be represented by an angle denoted asθ_(s).

The front wheel 112 and the rear wheel 110 of the wheelchair 100 areseparated by a distance d. The distance d between the two wheels may bemeasured as a linear (e.g., straight line) distance. The wheels 110 and112 typically have opposing counterparts (not shown) on the other sideof the wheelchair. The opposing counterparts may each share an axis withwheels 110 and 112, respectively. The area covered by the polygon whichconnects the points where these four wheels touch the ground (or theoutside portions of the ground contacting parts, when the groundcontacting part may cover more than a point) provides an area over whichthe center of gravity 104 may be located while the wheelchair remainsstable. This area may be referred to as the footprint of the device. Thefootprint of a device, as the term is used herein, is defined by theprojection of the area between the wheels as projected onto thehorizontal plane. If the center of gravity is above this location, thetransport device should remain stable.

If the center of gravity 104 is vertically displaced above the surface106 and outside the footprint (i.e., the projection of area between thewheels 110 and 112 onto the horizontal plane), wheelchair 100 may tipover. This could happen, for example, when the wheelchair is on asurface that has a steep incline, or, alternatively, if the user ‘pops awheelie’ in order to surmount a curb, for example. When on a steepincline, the center of gravity 104 may shift back and cause thewheelchair 100 to flip over backwards. This is shown in FIG. 1B wherethe center of gravity 104 is located at a position that is outside thefootprint of the wheelchair 100. The center of gravity 104 is shownincluding a gravity acceleration vector (g) which linearly translatesthe center of gravity 104 in a downward direction. The wheelchair 100may rotate about an axis of the rear wheel 110 until the wheelchair 100contacts the surface being traversed.

User 102 may help to return the center of gravity 104 to a location thatis above the area between the wheels 110 and 112 by leaning forward inthe wheelchair 100. Given this limited control of the location of thecenter of gravity 104, it is clear that human transport devices such aswheelchairs may encounter great difficulties when traversing unevensurfaces such as a curb or steps.

Some vehicles, by virtue of their weight distribution or typical modesof operation are prone to fore-aft instability and end-over-end (“endo”)rollovers. In operation of an all-terrain vehicle (ATV), for example, itis not always possible or desirable to maintain all wheels of thevehicle in contact with the underlying surface at all times. Yet, it isdesirable to preclude loss of control of the vehicle or end-over-endroll-over. ATVs may benefit from stabilization in one or more of thefore-aft or left-right planes, especially under conditions in whichfewer than a stable complement of wheels are in contact with the ground.Vehicles of this sort may be more efficiently and safely operatedemploying control modes supplementary to those described in the priorart.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the present invention, amethod is provided for fore-aft stabilization of a vehicle for motion ina specified direction over an underlying surface, where the vehiclehaving a plurality of driven wheels including a forward wheel and an aftwheel, and with the forward wheel characterized by a force normal to theinstantaneous direction of motion of the vehicle. The method has thestep of applying a torque to the aft wheel based on the force on theforward wheel normal to the direction of motion and/or theinstantaneously sensed tilt of the vehicle.

In accordance with other embodiments of the invention, the method mayhave additional steps of applying a periodic torque to at least one ofthe wheels for inducing a small pitch modulation, detecting pitchvariation of the vehicle in response to the applied periodic torque, andapplying a stabilizing torque to the aft wheel on the basis, at least,of any detected pitch variation in response to the applied periodictorque.

In accordance with yet further embodiments of the invention, anapparatus is provided for pitch stabilization of the motion of a vehiclehaving at least one forward wheel and at least one aft wheel. Theapparatus has a sensor for sensing a force on the forward wheel normalto an instantaneous direction of motion of the vehicle, a motor actuatorfor driving the aft wheel, and a controller for applying a torque to theaft wheel on the basis of a control law based at least on the normalforce on the forward wheel.

In accordance with another aspect of the present invention, a stabilizedvehicle is provided that has at least one forward wheel and at least oneaft wheel. Additionally, the vehicle has a sensor for sensing a force onthe forward wheel normal to an instantaneous direction of motion of thevehicle, and/or an instantaneous pitch of the vehicle and/or a functionof the instantaneous pitch. The vehicle also has a motor actuator fordriving the aft wheel, and a controller for applying a torque to the aftwheel on the basis of a control law based at least on the normal forceon the forward wheel. The vehicle may include a pedal-driven bicycle, amotorcycle, or a wheelchair.

In accordance with yet further embodiments of the present invention,there is provided a vehicle with a plurality of wheels, including atleast one forward wheel and at least one aft wheel. A motor actuatordrives each aft wheel, and a controller governs the motor actuator ormotor actuators in such a manner as to dynamically stabilize the vehiclewhen the forward wheel is not in contact with the underlying surface.More particularly, a left aft actuator drives a left aft wheelindependently of the right aft wheel, thus the controller can governdifferential rotation of the left and right aft wheels for controllingyaw of the vehicle whether a forward wheel is in contact with theground, or not.

In accordance with further embodiments of the invention, the controlleris such as to govern the motor actuator according to a control lawindependent of whether the forward wheel is in contact with theunderlying surface. The vehicle may further have a user input device forproviding a throttle output signal, and a pitch sensor for providing apitch signal. The controller may then govern the motor actuatoraccording to a control law based at least upon the throttle outputsignal or the pitch signal or a pitch rate signal. More particularly,the controller may govern the motor actuator according to a control lawbased at least upon the pitch signal when the vehicle pitch angleexceeds a specified value.

In accordance with a further embodiment of the invention, there isprovided a vehicle that includes a first fore-wheel coupled to a firstpivot point by a first strut and a second fore-wheel coupled to thefirst pivot point by a second strut. The vehicle of this embodiment alsoincludes at least one aft-wheel coupled to the first pivot point. Inthis embodiment, the first and second struts are spaced apart from oneanother and are arranged an configured to cause the vehicle to vary itsdirection of motion by causing the first fore-wheel and the secondfore-wheel to both pivot about at least their respective vertical axis.

In accordance with a further embodiment of the invention, there isprovided a vehicle that includes a central pivot. The vehicle alsoincludes a first fore-wheel coupled to the central pivot point by afirst strut, the first strut being arranged and configured to rotateabout the central pivot during operation and a second fore-wheel coupledto the central pivot by a second strut, the second strut being arrangedan configured to rotate about the central pivot during operation. Thevehicle of this embodiment also includes at least one aft-wheel coupledto the central pivot by a connecting member arranged configured toretain a fixed orientation with respect to the central pivot.

In accordance with a further embodiment of the invention, there isprovided a vehicle that includes a plurality of wheels, including atleast one forward wheel and at least two aft wheels. The vehicle of thisembodiment also includes at least one motor actuator that drives eachaft wheel and at least one yaw controller. The vehicle of thisembodiment also includes a controller that controls the at least onemotor actuator such that a direction imparted on the at least oneforward wheel by the yaw controller is replicated by differentialrotation of the at least two aft wheels.

In accordance with a further embodiment of the invention, there isprovided a vehicle that includes a plurality of wheels, including atleast one forward wheel and at least two aft wheels. The vehicle of thisembodiment also includes at least one motor actuator that drives eachaft wheel and at least one yaw controller. The vehicle of thisembodiment may also include a throttle and a controller that, when allof the plurality of wheels is in contact with a surface being traversed,causes the vehicle to accelerate when the throttle is rotated and that,when the at least one forward wheel is not in contact with a surfacebeing traversed, causes an offset from a pitch limit to be adjusted whenthe throttle is rotated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to thefollowing description, taken with the accompanying drawings, in which:

FIGS. 1A and 1B are schematic side views of a prior art personal vehicleof the type in which an embodiment of the invention may beadvantageously employed;

FIG. 2 is a diagram of typical components of a personal vehicle of thetype in which an embodiment of the invention may be advantageouslyemployed indicating the variables used in the description of specificembodiments of the present invention;

FIG. 3 is a block diagram depicting the coupling of pitch and yawcontroller outputs for generation of wheel amplifier commands;

FIG. 4A is a block diagram showing the constitutive inputs of a pitchcommand in accordance with an embodiment of the present invention;

FIG. 4B is a block diagram showing the constitutive inputs of a pitchcommand with a unilateral limit in accordance with an embodiment of thepresent invention;

FIG. 5A is a block diagram showing the constitutive inputs of a yawcommand in accordance with embodiments of the present invention;

FIGS. 5B and 5C are block diagrams of different embodiments of a yawcontroller in accordance with embodiments of the present invention;

FIG. 6 is a side view of an all-terrain vehicle capable of balancingoperation in accordance with one embodiment of the present invention;

FIG. 7 is a perspective view from above of the embodiment of theinvention of FIG. 6;

FIG. 8 is a further side view of the all-terrain vehicle of FIG. 6showing operation by a standing user;

FIG. 9 is yet a further side view of the all-terrain vehicle of FIG. 6showing operation by a seated user;

FIG. 10 shows the coupling of the handlebar to the upper pushrods forsteering of the front wheels in accordance with a preferred embodimentof the invention; and

FIG. 11 shows the coupling of the lower pushrods to steer the forwardwheels, in accordance with the embodiment of FIG. 10.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS DEFINITIONS

A vehicle may be said to act as “balancing” if it is capable ofoperation on one or more wheels but would be unable to stand on thosewheels alone, but for operation of a control loop governing operation ofthe wheels. A balancing vehicle, when operated in a balancing mode,lacks static stability but is dynamically balanced. The wheels, or otherground-contacting elements, that provide contact between such a vehicleand the ground or other underlying surface, and minimally support thetransporter with respect to tipping during routine operation, arereferred to herein as “primary wheels.” “Stability” as used in thisdescription and in any appended claims refers to the mechanicalcondition of an operating position with respect to which the system willnaturally return if the system is perturbed away from the operatingposition in any respect. The term “system” refers to all mass caused tomove due to motion of the wheels with respect to the surface over whichthe vehicle is moving, and thus includes both the vehicle and the rider.

The term “lean” is often used with respect to a system balanced on asingle point of a perfectly rigid member. In that case, the point (orline) of contact between the member and the underlying surface has zerotheoretical width. In that case, furthermore, lean may refer to aquantity that expresses the orientation with respect to the vertical(i.e., an imaginary line passing through the center of the earth) of aline from the center of gravity (CG) of the system through thetheoretical line of ground contact of the wheel. Recognizing that thetire of an actual wheel is not perfectly rigid, the term “lean” is usedherein in the common sense of a theoretical limit of a rigidground-contacting member.

OPERATION IN ACCORDANCE WITH EMBODIMENTS OF THE INVENTION

One embodiment of a stabilized vehicle in accordance with the presentinvention is depicted in FIG. 2 and designated generally by numeral 10.User 8, as shown in FIG. 2, is seated position on user support 12 ofvehicle 10, though it is to be understood that user 8 may be supportedotherwise than by sitting on a seat, and may, for example, within thescope of the present invention, be standing on a user support in theform of a platform.

Aft wheels 21 (of which only one is visible in the side-view of FIG. 2)are coaxial about an axis defined as the Y axis. Each of rear wheels 21is driven by a motor actuator (not shown) disposed within a power base24 such that steering may be effectuated through differential torqueapplied to respective rear wheels 21. Compensating, by differentialactuation of the rear wheels for the increased rotational travel of theouter wheel on a turn may be referred to herein as an “activedifferential.” Rider 8 may be supported on vehicle 10 in various bodypositions, thereby controlling the position of the center of mass of thevehicle, as governed by the distribution of weight of the load, namelythe user. For example, user 8 may be seated, as shown in FIG. 2, on seat12, with his feet resting on footrest 26.

The embodiment shown of vehicle 10, additionally, has two forward wheels13 (of which one is visible in the side view of FIG. 2), typically incontact with the ground during ordinary operation. Forward wheel 13 andone or more other forward wheels may be mounted on a common axle orotherwise, and pivoting of any of the forward wheels is within the scopeof the present invention. Personal vehicles designed for enhancedmaneuverability and safety may also include one or more clusters ofwheels, with the cluster and the wheels in each cluster capable of beingmotor-driven independently of each other. Such vehicles are described inU.S. Pat. Nos. 5,701,965, 5,971,091, 6,302,230, 6,311,794, and6,553,271, all of which patents are incorporated herein by reference.

Controller 30 provides for stability of the vehicle by continuouslysensing the orientation of the vehicle and the commanded velocity, asdescribed in detail below, determining the corrective action to maintainstability, and commanding the wheel motors to make any necessarycorrective action.

In accordance with preferred embodiments of the present invention, thesame control law is applied whether or not forward wheels of the vehicleare in contact with the ground.

Steering or other control may be provided by means of a user inputdevice 18, which may be a joystick, handlebars or by any other userinput mechanisms. A variety of steering devices which are furtherexamples of user input mechanisms that may be employed within the scopeof the present invention are described in U.S. Pat. Nos. 6,581,714 and6,789,640, which are incorporated herein by reference.

A sensor unit 28 is provided as part of power base 24 for providing oneor more sensor signals to controller 30. Sensor unit 28 may provide ameasure of pitch rate and/or pitch of the vehicle, and may employinertial sensing of the type described in detail in US Pat. No.6,332,103, which is incorporated herein by reference. Alternatively, oradditionally, sensor unit 28 may include a force sensor for measuringthe force (designated by arrow 32) normal to the underlying surface thatis exerted on the underlying surface by wheel 13 (and, reciprocally, onthe wheel by the underlying surface). Force sensors, such as those basedon piezoresistors, are well-known in the art, and any kind of forcesensor is within the scope of the present invention.

A simplified control algorithm for achieving balance in the embodimentof the invention according to FIG. 2 is now described. The controlalgorithm is described for the case of a single driven wheel, as may beemployed for stabilization of an in-line bicycle or motorcycle. Thegeneralization to the case of multiple driven wheels is discussed indetail below.

To achieve dynamic control to insure stability of the system, the wheeltorque T in this embodiment is governed by the following simplifiedcontrol equation:T=K ₁(θ)·(θ−θ₀)+K ₂ ·θ+K ₃·(v−v _(command))+K ₄·∫(v−v_(command))dt+A·f(ωt),   (Eqn. 1)where:

T denotes a torque applied to a ground-contacting element about its axisof rotation;

K₁(θ) is a gain function that may depend, as discussed below, on theinstantaneous value of lean θ;

θ is a quantity corresponding to the lean of the entire system about theground contact region beneath the common axis Y of the rear wheels, withθ₀ representing the magnitude of a system pitch offset, all as discussedin detail below;

v identifies the fore-aft velocity along the surface, with v_(command)representing the magnitude of a user input such as a throttleconstituted by user input (e.g., joystick) 18;

a dot over a character denotes a variable differentiated with respect totime; and

a subscripted variable denotes a specified offset that may be input intothe system as described below; and

K₁, K₂, K₃ and K₄ are gain functions or coefficients that may beconfigured, either in design of the system or in real-time, on the basisof a current operating mode and operating conditions as well aspreferences of a user. The gain coefficients may be of a positive,negative, or zero magnitude. The gains K₁, K₂, K₃ and K4 are dependentupon the physical parameters of the system and other effects such asgravity. The simplified control algorithm of Eqn. 1 maintains balance ofthe vehicle in the presence of changes to the system's center of massdue to body motion of the rider or features of the underlying terrain.

The final term of Eqn. (1) allows for application of a periodic drivingcomponent, of period 2π/ω, and amplitude A (which may be zero, in thecase of no applied modulation), to the torque applied to the drivenwheel. The periodic function f(ωt) may be a sinusoidal function, forexample.

It should be noted that the amplifier control may be configured tocontrol motor current (in which case torque T is commanded) or,alternatively, the voltage applied to the motor may be controlled, inwhich case the commanded parameter is velocity.

The effect of θ₀ in the above control equation (Eqn. 1) is to produce aspecified offset θ₀ from the non-pitched position, θ=0. Adjustment of θ₀will adjust the vehicle's offset from a non-pitched position. In someembodiments, pitch offset may be adjusted by the user. Alternatively, θ₀can be set by the control system of the vehicle as a method of limitingthe speed and/or the performance of the vehicle. In a preferredembodiment of the invention, a backward tilting limit is imposed, withthe gain function K₁ substantially zero until the tilting limit isapproached. Thus, the rider is free to lean the vehicle backward byshifting his weight, and thus the center-of-mass of the vehicle system,backward, until the tilt limit is approached. Then, K₁ assumes anon-zero value, and a term appears in control equation (1) that tends tocounteract further backward leaning of the vehicle.

The magnitude of K₃ determines the gain of the user input, and mayadvantageously be a non-linear function, providing, for example, greatersensitivity near zero velocity. The K₂ term provides for control basedon the instantaneous pitch rate, {dot over (θ)}, of the vehicle, asmeasured by a pitch rate sensor or by differentiation of a measuredpitch.

The response of a normal force 32 measured by sensor 28 in response tothe applied pitch modulation Af(ωt), may be used, in accordance withembodiments of the invention, to counteract further backward leaning ofthe vehicle and maintain contact of the forward wheel with the ground,or, alternatively, impose a limit on rearward tilt.

In order to accommodate two wheels instead of the one-wheel system thathas been described with respect to Eqn. 1, separate motors may beprovided for left and right wheels of the vehicle and the torque desiredfrom the left motor and the torque to be applied by the right motor canbe governed in the general manner described above. Additionally,tracking both the left wheel motion and the right wheel motion permitsadjustments to be made to prevent unwanted turning of the vehicle and toaccount for performance variations between the two drive motors.

In accordance with preferred embodiments of the invention, differentialdrive of the two rear wheels tracks turns according to the same yawinput as applied, via mechanical linkages, to the front wheels. Thisoperation is described below, with reference to FIGS. 10 and 11.

Referring now to FIG. 3, steering, or yaw control, of the vehicle may beaccomplished by adding a turning command to the wheel amplifiers andhave the following form. Inputs (described below) corresponding tovalues of vehicle parameters are used by Pitch Controller 500 and YawController 502 to derive a balance control signal BalCmd and a yawcontrol signal YawCmd according to algorithms discussed in thesucceeding paragraphs.LeftCmd=BalCmd+YawCmd (2)RightCmd=BalCmd−YawCmd (3)The LeftCmd and RightCmd are the command sent by the controllers 500 and502 to the left and right motor amplifiers, respectively, afterdifferentiation or other conditioning as appropriate. For instance andas shown by way of example in FIG. 3, the LeftCmd and RightCmd's may beconditional, respectively, by differentiators 504 and 506. The LeftCmdand RightCmd represent voltage if the amplifiers are in voltage controlmode, current if the amplifiers are in current control mode, or dutycycle if the amplifiers are in duty cycle control mode. BalCmd is thecommand sent by the Pitch Controller 500 to each amplifier to maintainthe transporter in a balanced state while moving or while at rest. TheYawCmd causes the transporter to turn by reducing the command to one ofthe wheels while increasing the command to the other wheel. For example,a positive YawCmd increases the command to the left wheel whiledecreasing the command to the right wheel thereby causing thetransporter to execute a right turn. The YawCmd may be generated by ayaw-input device described above with no feedback loop or in a closedcycle loop to correct yaw position errors as described in U.S. Pat. No.6,288,505.

Pitch controller 500 is described in detail with reference to FIGS. 4Aand 4B. The inputs include a desired pitch θ_(desired), the actualmeasured pitch θ, the pitch rate {dot over (θ)}, and the component ofthe wheel rotation velocity that is common to the two primary wheels,ω_(com). Both θ and {dot over (θ)} may be derived from inertial sensing,as described in U.S. Pat. No. 6,332,103, which is incorporated herein byreference.

Desired pitch θ_(desired) and current instantaneous pitch θ aredifferenced in summer 520 to produce a pitch error θ_(err). Inaccordance with certain embodiments of the present invention, pitchlimiting is unilateral, such that a limit is provided on one end of arange of allowed values of pitch. If that pitch is exceeding, arestoring torque moves the vehicle in the direction of the pitch limit.

In accordance with some embodiments of the invention, the user may shifther weight backward, thereby ‘popping’ the vehicle into a two-wheeledbalancing condition where stability is maintained until she shifts herweight forward to restore operation on all wheels.

A term quadratic in pitch error θ_(err) (preserving the sign of theactual pitch error) may also be provided, as shown in FIG. 4B, therebyproviding more intense response to large deviations in pitch as mayresult from encountering an obstacle, for example. In a voltage controlmode, it is desirable to provide an additional term proportional to thewheel rotational velocity to compensate for all, or a portion, of theback-emf generated in proportion to the rotational velocity of themotors.

Yaw controller 502 is described in detail with reference to FIGS. 5A-5C.FIG. 5A depicts the differencing, in summer 522, of the current yawvalue ψ with respect to the desired yaw value ψ_(desired) to obtain thecurrent yaw error ψ_(err). Desired yaw value ψ_(desired) is obtainedfrom a user input such as joystick 18 or other user input deviceemployed for directional input as discussed above. The current value ofyaw is derived from various state estimates, such as the differentialwheel velocities, inertial sensing, etc. Derivation of the yaw commandfrom the yaw error is provided by controller 524 according to variousprocessing algorithms.

Two examples of yaw control algorithms are shown in FIGS. 5B and 5C.Specifically, FIG. 5B shows a control law implemented input signalψ_(err) is added, by summer 560, to the derivative of itself (output ofdifferentiator 562) and the intergration of itself (output of integrator564). Of course, and as shown by in FIG. 5B, each signal could have again applied to it (for example, by gain blocks 568, 569, and 570) orother signal processing such as smoother 566.

Another possibility is to simply omit the derivative signal as shown inFIG. 5C.

Of course, various controller transfer strategies may be implementedwith proportional, derivative, and ‘three term’ ‘PID’ functions asdepicted.

The present invention may also be embodied in a balancing all-terrainvehicle as depicted in FIG. 6 and designated generally by numeral 10.User 8, as shown in FIG. 6, is in a seated position on user support 12of all-terrain vehicle 10. Aft wheels 21 and 22 are shown as coaxialabout an axis defined as the Y axis. Referring now to the perspectiveview of all-terrain vehicle 10, from the top, shown in FIG. 7, each ofrear wheels 21 and 22 is driven by a motor actuator 24 such thatsteering may be effectuated through differential torque applied to ofrear wheels 21 and 22. Compensating, by differential actuation of therear wheels for the increased rotational travel of the outer wheel on aturn may be referred to herein as an “active differential.” Rider 8 maybe supported on vehicle 10 in various body positions, therebycontrolling the position of the center of mass of the vehicle, asgoverned by the distribution of weight of the load, namely the user. Forexample, user 8 may be seated, as shown in FIG. 6, on seat 12, with hisfeet resting on platform 26 (shown in FIG. 7), and may shift his weightrelative to the vehicle by positioning himself along the length of seat12. Alternatively, user 8 may stand on platform 26, with legs athwartseat 12, as shown in FIG. 8, or may sit on seat 12 with feet resting onfoot rests 28, as shown in FIG. 9.

Referring again to FIG. 6, the embodiment shown of vehicle 10,additionally, has two forward wheels, 13 and 14, typically in contactwith the ground during ordinary operation. In the embodiment of theinvention shown, by way of example in FIG. 7, each forward wheel 13 and14 is mounted on a separate suspension strut 29 such that each forwardwheel is suspended independently of one another.

Controller 30 (shown in FIG. 7) provides for stability of the vehicle bycontinuously sensing the orientation of the vehicle and the commandedvelocity, as has been described above, determining the corrective actionto maintain stability, and commanding the wheel motors to make anynecessary corrective action. In accordance with preferred embodiments ofthe present invention, the same control law may be applied whether ornot forward wheels 13 and 14 of the vehicle 10 are in contact with theground.

Steering or other control may be provided by the user's rotation ofhandlebar 18 (shown in FIG. 7) about pivot 17, or by any other userinput mechanisms. A variety of steering devices which are furtherexamples of user input mechanisms that may be employed within the scopeof the present invention are described in U.S. Pat. Nos. 6,581,714 and6,789,640, which are incorporated herein by reference. Handlebar 18 mayalso support user instruments and other user controls such as athrottle, within the scope of the invention.

In operation of a vehicle that may operate on either two or four wheels,it may be beneficial that if a turn is initiated in one mode, it besmoothly continued, either as wheels leave the ground or as wheelsremake ground contact. To that end, in accordance with preferredembodiments of the invention, a mechanical linkage is provided betweenthe user yaw input and the forward wheels, while the rear wheels arecontrolled, in synchrony with any turn initiated by the user input, bymeans of differential rotation of the wheels. Referring to FIG. 10,vehicle steering is implemented, in accordance with the embodimentshown, by turning handlebar 18 about pivot 17. This serves twofunctions: steering the forward wheels, and providing electrical inputto cause differential rotation of the aft wheels. To steer the forwardwheels, motion is transferred, via bellcrank 80, to fore-aft axialmotion of upper push rods 82, as indicated by arrows 83 in FIG. 7.Bellcrank 80 is a lever with two arms forming a fixed angle betweenthem, and a fulcrum at the apex of the angle. This allows rotationalmotion (of the handlebar pivot) substantially transverse to the groundto be transferred to motion (of the upper push rods) having asignificant component parallel to the ground. Upper push rods 82, inturn, via middle bellcranks 84, transfer motion to lower pushrods 90,shown in FIG. 11, which turn forward wheels 13 and 14 by causing them topivot about vertical pivot axes 92. It is to be understood that anyother couplings, mechanical or motorized, between the user input and theangle of forward wheels 13 and 14, may also be employed within the scopeof the present invention.

At the same time that the user yaw input, such as the handlebar, governsthe steering of the forward wheels as described above, a signal isgenerated, by means of a rotational transducer, or otherwise, to serveas the input to yaw controller 502 (shown in FIG. 3) to governdifferential actuation of the rear wheels. Thus, the user-intendedsteering is accomplished, in accordance with this invention, whether ornot the forward wheels are in contact with the ground. Various means ofconverting the mechanical user input (such as handlebar rotation angle)to a yaw signal input to the controller 70 are known in the art, such asthose described in U.S. Pat. No. 6,581,714, for example, and any suchmeans are encompassed within the scope of the present invention.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

1. A method for fore-aft stabilization of a vehicle for motion in aspecified direction over an underlying surface, where the vehicle havinga plurality of driven wheels including a forward wheel and an aft wheel,and with the forward wheel characterized by a force normal to theinstantaneous direction of motion of the vehicle, the method comprising:applying a torque to the aft wheel based on the force on the forwardwheel normal to the direction of motion.
 2. A method according to claim1, further comprising: applying a periodic torque to at least one of thewheels for inducing a small pitch modulation.
 3. A method according toclaim 2, further comprising: detecting pitch variation of the vehicle inresponse to the applied periodic torque; and applying a stabilizingtorque to the aft wheel on the basis, at least, of any detected pitchvariation in response to the applied periodic torque.
 4. A method forfore-aft stabilization of a vehicle for motion in a specified directionover an underlying surface, where the vehicle having a plurality ofdriven wheels including a forward wheel and an aft wheel, and with theforward wheel characterized by a force normal to the instantaneousdirection of motion of the vehicle, the method comprising: applying atorque to the aft wheel based on the force on the forward wheel normalto the instantaneously sensed tilt of the vehicle.
 5. A method accordingto claim 4, further comprising: applying a periodic torque to at leastone of the wheels for inducing a small pitch modulation.
 6. A methodaccording to claim 5, further comprising: detecting pitch variation ofthe vehicle in response to the applied periodic torque; and applying astabilizing torque to the aft wheel on the basis, at least, of anydetected pitch variation in response to the applied periodic torque. 7.An apparatus for pitch stabilization of the motion of a vehicle havingat least one forward wheel and at least one aft wheel, the apparatuscomprising: a sensor for sensing a force on the forward wheel normal toan instantaneous direction of motion of the vehicle; a motor actuatorfor driving the aft wheel; and a controller for applying a torque to theaft wheel on the basis of a control law based at least on the normalforce on the forward wheel.
 8. An apparatus according to claim 7,further comprising a second sensor for sensing a force on the forwardwheel normal to the instantaneous direction of motion of the vehicle. 9.An apparatus according to claim 7, wherein the controller causes themotor actuator to apply a periodic torque to the at least one aftwheelto induce a small pitch modulation.
 10. An apparatus according to claim9, further comprising: a pitch detector to detect a pitch variation ofthe vehicle in response to the periodic torque; wherein the controllercauses the motor actuator to apply a stabilizing torque to the at leastone aft wheel on the basis, at least, of any detected pitch variation inresponse to the applied periodic torque.
 11. A stabilized vehiclecomprising: at least one forward wheel; at least one aft wheel; at leastone sensor for sensing a force on the forward wheel normal to aninstantaneous direction of motion of the vehicle; at least one sensorfor sensing an instantaneous pitch of the vehicle; a motor actuator fordriving the aft wheel; and a controller for applying a torque to the aftwheel on the basis of a control law based at least on the normal forceon the forward wheel.
 12. A stabilized vehicle according to claim 11,wherein the vehicle is a pedal-driven bicycle.
 13. A stabilized vehicleaccording to claim 11, wherein the vehicle is a motorcycle.
 14. Astabilized vehicle according to claim 11, wherein the vehicle is an allterrain vehicle.
 15. A stabilized vehicle according to claim 11, whereinthe vehicle is a wheelchair.
 16. A stabilized vehicle according to claim11, wherein the controller controls operation of the vehicle in the samemanner regardless of whether the forward wheel is in contact with asurface being traversed.
 17. A stabilized vehicle comprising: at leastone forward wheel; at least one aft wheel; at least one sensor forsensing an instantaneous pitch of the vehicle; a motor actuator fordriving the aft wheel; and a controller for applying a torque to the aftwheel on the basis of a control law based at least on a function of theinstantaneous pitch of the vehicle.
 18. A stabilized vehicle accordingto claim 17, wherein the vehicle is a pedal-driven bicycle.
 19. Astabilized vehicle according to claim 17, wherein the vehicle is amotorcycle.
 20. A stabilized vehicle according to claim 17, wherein thevehicle is an all terrain vehicle.
 21. A stabilized vehicle according toclaim 17, wherein the vehicle is a wheelchair.
 22. A stabilized vehicleaccording to claim 17, wherein the control law is based at least on theinstantaneous pitch of the vehicle.